Neodymium salts activated with aluminum alkyl co-catalysts have been known to catalyze the polymerization of conjugated dienes since the early 1960's. To date, many papers and patents have been published which describe variations and improvements to the original systems (see U.S. Pat. No. 3,297,667, U.S. Pat. No. 3,676,441, and U.S. Pat. No. 3,794,604). Much of this work was driven by the eventual commercialization of high cis-polybutadiene in the 1980s for use in tire applications (see U.S. Pat. No. 4,242,232, U.S. Pat. No. 4,260,707, U.S. Pat. No. 4,699,960, and U.S. Pat. No. 4,444,903).
The type of catalyst system employed and its method of preparation are crucial to the success of the polymerization. Traditionally, there are two main types of catalyst systems. The first is a ternary system based on soluble neodymium carboxylates in conjunction with an alkylaluminum co-catalyst and a halogen source (see R. P. Quirk, A. M. Kells, K. Yunlu, J. P. Cuif, Polymer 41, 5903 (2000) and A. Pross, P. Marquardt, K. H. Reichert, W. Nentwig, T. Knauf, Angew. Makromol. Chem. 211, 89 (1993)). The second system is a binary catalyst comprising an insoluble neodymium halide complexed with three equivalence of a Lewis base such as an alcohol, amine, or phosphonate and an alkylaluminum activator (see H. Iovu, G. Hubca, E. Simionescu, E. Badea, J. S. Hurst, Eur. Polymer J. 33, 811 (1997); H. Iovu, G. Hubca, D. Racoti, J. S. Hurst, Eur. Polymer J. 35, 335 (1999); and J. H. Yang, M. Tsutsui, Z. Chen, D. Bergbreiter, Macromolecules 15, 230 (1982)).
In general, the two systems behave similarly; however, the ternary system appears to have gained acceptance commercially in the production of polybutadiene (see D. J. Wilson, J. Polym. Sci., Part A. 33, 2505 (1995)). Typically, the most active ternary systems consist of treating a branched long chain neodymium carboxylate with branched trialkyl-aluminum or dialkylaluminum hydrides, in an Al/Nd ratio between 10–40/1, and the use of 2–3 equivalents of a halide source, such as diethylaluminum chloride or tert-butylchloride.
The active catalyst is typically prepared in one of two ways. The simplest method is to generate the catalyst in-situ by sequentially introducing the catalyst components to the polymerization solution. It is usually most effective to introduce the aluminum alkyl components first, thereby scavenging impurities from the premix prior to contact with the neodymium salt. The other method for catalyst preparation is to preform the catalyst components prior to introducing them into the polymerization vessel. The most common practice involves sequentially treating the catalyst components in the presence of at least a few equivalents of monomer followed by an aging period. For example U.S. Pat. No. 3,794,604 discloses an improved preforming technique which is carried out in the presence of a small portion of a conjugated diene.
Aging the catalyst components with a diene prior to polymerization results in a more active catalyst then when the conjugated diene is absent. The disclosed technique for catalyst formation is to age after all of the components have been mixed together. U.S. Pat. No. 4,429,089 also teaches the use of a diolefin during catalyst formation and states that the particular procedure which is followed has no bearing on the polymerization run. Likewise, U.S. Pat. No. 4,461,883 discloses that the use of a conjugated diene in the catalyst make-up is preferable for improving the activity of the catalyst. In this example, the diene is mixed with the catalyst components at any time in the preforming step with aging occurring after all components are mixed together.
U.S. Pat. No. 4,533,711 teaches the practice of adding the catalyst components together first followed by the addition of a small amount of diene and then aging the preformed catalyst. This patent states that the diene is not essential in the make-up but it does serve to increase catalyst activity. U.S. Pat. No. 4,663,405 continues to teach the use of conjugated dienes as components in preformed catalysts. It goes on to state that soluble catalysts result when diolefins are present in the make-up while insoluble catalysts frequently result when no diene is present. This patent teaches a process where aging of the catalyst occurs after the reagents are added.
U.S. Pat. No. 5,502,126 again practices the use of a diene in the preformed catalyst make-up and again states that it is preferred to age the catalyst after the labile halogen compound is added. In U.S. Pat. No. 5,659,101 the use of a diolefin in conjunction with a boron derived halogen source results in a preformed catalyst that partially forms a solid precipitate in aliphatic solvents.
When silicone halides are used, as in U.S. Pat. No. 5,686,371, aging in the presence of a diene again is performed after the addition of all the catalyst components. U.S. Pat. No. 6,136,931 discloses an improved boron halide dependent preformed catalyst that has excellent solubility in non-polar solvents. Finally, U.S. Pat. No. 6,255,416 also practice preformed catalyst generation in the presence of a small amount of diene. Aging in this case again occurs after all of the catalyst components are mixed.